1. Field of the Invention
This invention relates generally to ball-and-socket joints and, in particular, to implantable prostheses. More particularly, the invention relates to an implantable prosthetic socket having a bearing insert interposed between the spherical head of an associated prosthetic component and the socket component of the prosthesis.
2. Description of the Prior Art
Joint replacements--total and partial--have in recent years become reliable orthopedic procedures for the correction of disorders of natural joints. These procedures when applied to a hip joint, for example, involve the surgical implantation of a femoral stem prosthesis into the femur and an acetabular endoprosthesis into the acetabulum for receiving the femoral head. Similar procedures and components may be used with other ball-and-socket type joints such as, for example, a shoulder joint, although the preferred embodiment disclosed herein is described in terms of a hip joint. Additionally, while the preferred embodiment is described in terms of an implantable prosthetic socket, those skilled in the art will understand that the invention may be applied to ball-and-socket joints in general, whether intended to be implantable or not and indeed may be applied to other than "spherical" ball-and-socket joints.
Numerous devices have been developed in an effort to produce a prosthetic hip implant which is not only easy to implant or remove, if necessary, but which also approximates as closely as possible the performance of the natural joint. While all known devices have a femoral and an acetabular component, the design of the latter varies widely depending upon the dysfunction being corrected. The acetabular socket component is generally a biocompatible metallic member that may, for example, be either movably or immovably secured into the acetabulum. In either case, the acetabular socket has a low-friction biocompatible bearing insert (generally polyethylene) with a seat or socket chamber for receiving the spherical head or ball-end of the femoral component.
Those devices which have a fixed acetabular component have a rigid, cup-shaped socket component intended to be retained within the natural acetabular socket. They have a single articulating surface--that is, the head of the femoral component is movable within the bearing insert. Another class of devices is known wherein the acetabular socket component is movable within the natural socket. These, devices have a rigid, cup-shaped outer shell with a highly polished outer spherical surface designed to move within the natural socket. These devices have two articulating surfaces--the head and the outer surface of the socket--and are generally known as "bipolar", including both concentric and non-concentric designs, the latter having varying degrees of eccentricity or polar offset.
An important consideration in the design of bipolar hip endoprostheses is enhancement of the prosthesis to resist the tendency of the acetabular cup of these implants to rotate toward varus. Producing bipolar endoprostheses with an antivarus property (i.e. polar offset) is desirable in order to advantageously distribute the inherent forces which tend to deform the implant's components and, in the extreme, result in dislocation. The general manner in which an antivarus property is effected is by providing a polar offset. Ball-and-socket type joints are subject to dislocation under a certain types or amounts of stress and producing these joints (for hip applications) with a polar offset between the inner and outer articulating components tends to decrease these stresses, thereby decreasing the tendency of these components to come apart. It is a goal of all ball-and-socket type prostheses to overcome this tendency by more securely capturing the femoral head within the bearing insert. It is a particular object of this invention to provide a socket which strongly captures the femoral head over a wide range of polar offsets (to accomodate prostheses of various sizes).
The importance of securely capturing the head in a socket must be considered in the light of other desirable factors such as ease of assembly and installation and ease of disassembly. Occasionally, due to subsequent injury or disease, the replacement of all or part of the prosthesis becomes necessary. Consequently, while such devices are often designed with the intention of separating the various components if necessary, simultaneous consideration must be given to making sure the assembled components do not easily become separated during normal use. Numerous prior art devices are known which disclose many ways of accomodating ease of assembly, resistance to dislocation and ease of disassembly in prosthetic ball-and-socket joints.
U.S. Pat. No. 3,813,699 (Giliberty) discloses an outer shell with a bearing insert immovably secured to the shell. The insert has a constricted opening and is of a resilient silicone material capable of yielding sufficiently to enable the head of the femoral stem prosthesis to be inserted into the socket of the insert while thereafter capable of retaining the head in an articulating manner.
U.S. Pat. No. 3,818,512 (Shersher) discloses a prosthesis having a conventional, fixed acetabular component. The device includes an outer metal shell and a bearing insert having a plurality of fingers or leaves intended to be compressed inwardly to retain the head of the femoral component. The compression is achieved by a self-locking nut which threads into the outer metal shell, the inner surface of the nut being shaped to conform to the fingers of the bearing insert. The nut has several lugs to facilitate the threading of the nut into the acetabular component and is locked to the outer shell by a partial circumferential slot splitting the nut into two parallel parts which are then compressed together by a screw. While the Shersher device includes a locking element to more positively secure the head of the femoral prosthesis to the acetabular shell, this locking mechanism--even if it were adapted for use in a bipolar prosthesis--is difficult to use in practice because of the required threaded motion.
U.S. Pat. No. 3,863,273 (Averill) discloses a similar type of locking or capture mechanism to secure the head of the femoral component into the bearing insert. The bearing insert of Averill is secured within the outer shell by a radially outwardly extending tab on the insert snapping into a corresponding annular groove on the inner wall of the outer shell. The open end of the bearing insert is a greatly constricted opening having longitudinal slots defining fingers which are radially expanded to enlarge the opening when the head is pushed in. The Averill device relies only on the tension of the fingers to hold the femoral head, the fingers being supported by the surrounding inner surface of the outer shell, and does not disclose any other positive locking mechanism to secure the head of the femoral component within the bearing insert. As will be understood by those skilled in the art, this structure and other similar types of structures described below necessarily require a large amount of plastic bearing insert material between the head and the opening of the metal shell. This contributes to deformation of the bearing insert and subsequent dislocation under high loads. The Averill device does not disclose any specific means for dissassembling the femoral head from the bearing insert.
U.S. Pat. No. 4,784,663 (Kenna) discloses a structure similar to Averill except that the resilient fingers or leaves of the bearing insert are prevented from expanding radially outwardly by a polyethylene locking ring interposed between the open end of the bearing insert and the open end of the outer shell and retained in place by the cooperative action of an annular rib and groove attaching mechanism. The Kenna device facilitates disassembly by having an annular gap between the lip of the locking ring and the adjacent periphery of the metal shell. The locking ring may be pulled apart from the bearing insert by prying the lip away from the shell periphery using a suitable tool inserted in the annular gap.
U.S. Pat. No. 4,770,658 (Geremakis) discloses another type of locking ring in a joint prosthesis. In this device a bearing insert is provided with a plurality of axially extending fingers designed to be pressed radially inwardly by a lock ring interposed between the fingers and the outer metal shell. The lock ring is carried by the outer shell in a first position enabling the insertion of the head of an associated component and a second position in which the lock ring tightly engages the bearing insert and the outer shell to maintain the bearing insert in overlapping relation to the head. Unlocking is achieved by pulling the lock ring axially away from the outer shell (a part of the rim of the shell is cut-out for this purpose).
U.S. Pat. No. 4,241,463 (Khovaylo) discloses a prosthetic hip implant wherein the femoral head is captured within the spherical cavity of a bearing insert by a split polyethylene retaining ring as opposed to a solid ring, the ring being interposed between the head and the inside of the insert (rather than the inside of the outer shell). The insert is keyed to prevent its rotation relative to the outer shell and the split ring is adapted to move within a cavity of the bearing insert within a limited axial range. Upon insertion of the femoral head into the bearing insert, the split ring is forced axially to one extreme end of its axial range of motion where the cavity is larger so the ring may expand radially to enable it to open up sufficiently to permit the head to enter the spherical seat of the insert. When the head is properly seated, it clears the split ring sufficiently to enable the ring to return to its normal size underneath the femoral head. The interior cavity of the bearing insert and the split ring are so angled that attempted removal of the femoral head tends to wedge the split ring against the interior of the bearing insert thereby preventing removal of the femoral head under normal circumstances. A slot is provided adjacent the opening of the bearing insert to enable a tool to be inserted in order to push the split ring inwardly to its expanded position in order to enable the femoral head to be removed.
U.S. Pat. No. 4,380,090 (Ramos) discloses a hip prosthesis with a split annular polyethylene bearing similar to Khovaylo, but adjacent the open end of a bearing insert between the head and the outer shell. The split bearing is retained by split metal locking ring held within an inner annular groove on the outer shell. Removal of the bearing insert is accomplished by compressing the metal locking ring to remove it, the bearing insert and the split annular bearing.
U.S. Pat. No. 4,619,658 (Pappas et al.) discloses various prosthetic hip implants, some having solid bearing rings and some having split bearing rings. In one embodiment, the body of the bearing insert is formed from a plurality of segments (split in a plane passing through and parallel to the axis) which are held together by a circular retaining ring and which are held within an outer metal shell by the cooperative action of the retaining ring with a groove on the inner surface of the outer shell. In another embodiment, the bearing insert is formed of two pieces split along a transverse plane: a primary bearing insert forming the major part of the spherical head of the insert and a split collar (i.e. bearing ring) for being attached to the open end of the primary bearing insert. The collar and the primary bearing insert are held together by the interaction of cooperative annular ribs and grooves and the assembly is then inserted into an outer metal shell. A substantially circular release tool may be inserted into the open end of the outer shell between it and the bearing insert to compress the ring of the first embodiment to remove the bearing insert from the outer metal shell. In the second embodiment the collar may be provided with holes or slots into which the ends of conventional clamps may be inserted to squeeze the collar together thereby reducing its circumference at the radial split in order to enable the bearing insert to be removed from the outer shell.
U.S. Pat. No. 4,714,477 (Fichera et al.) discloses another version of a split bearing ring in a single-piece bearing insert having an upper circumferential portion and a lower portion integrally connected to the upper portion but separated from it by a circumferential split (extending less than 360.degree.). The lower portion itself is split along a vertical line and is retained within the outer shell by an annular rib engaged within an annular groove of the shell.
All known ball-and-socket devices appear to suffer from the inherent limitation that, under extreme ranges of motion where the loads may be expected to be greatest, plastic flow may occur in the bearing insert (and other plastic components). This is especially true of those implantable devices subject to rotate toward varus so that the forces tend to be directed against the rim of the cup and insert. Even in embodiments which may not be subject to rotation toward varus (as in, for example, non-implanted joints) the forces causing dislocation of the head from the spherical cavity are, in many prior art designs, directed primarily against that portion of the bearing insert and/or bearing ring adjacent the open end of the outer cup. That is, the forces are directed against a portion of the bearing insert which is not supported by much more than the plastic mass of the bearing insert itself. Increasing the mass of bearing insert material has not been of any particular advantage in increasing the resistance of the prior art sockets to dislocation.
It is accordingly an object of this invention to produce an improved socket for use in ball-and-socket joints to provide increased resistance to dislocation.
It is another object of this invention to produce an implantable prosthetic socket wherein the head of an associated femoral stem prosthesis is captured within the socket of a bearing insert by a substantially annular bearing ring.
It is a further object of this invention to produce an implantable prosthetic socket joint capable of being easily assembled and disassembled. It is in part an object of this invention to facilitate the assembly and disassembly operations by minimizing the number of components and subassemblies required to produce an operational prosthetic joint.
It is yet another object of this invention to produce an implantable prosthetic socket having an outer shell and an inner bearing insert and utilizing a split bearing ring to form part of the socket chamber within the bearing insert, the prosthetic socket adapted to have the split bearing ring being directly in contact with both the head of an associated prosthetic component and the inner surface of the outer shell along an interior portion thereof distal from the opening.
It is also an object of this invention to produce a unique bipolar endoprosthesis adaptable to a range of eccentricities while also maximizing the ability of the assembled components to resist dislocation.